1. Field of the Invention
The present invention relates to a nitride semiconductor device such as a light emitting device like a laser diode (LD) and a light emitting diode (LED), a light receiving device like a photodiode, and an electronic device like a transistor. More particularly, the present invention relates to a nitride semiconductor device having a p-type layer.
2. Related Art
A nitride semiconductor has been used as a material of a light emitting device such as a laser diode and a light emitting diode.
FIG. 11 is a cross-sectional view of a conventional nitride semiconductor functioning as a laser device, which is described in Japanese Laid-Open Patent Publication No. 2001-148357. As shown in FIG. 11, in the conventional semiconductor device, an n-type GaN (gallium nitride) layer 103, an n-type cladding layer 104 made of n-type AlGaN (aluminum gallium nitride), an n-type optical guide layer 105 made of n-type GaN, a multiple quantum well layer 106 made of InGaN (indium gallium nitride), an electron blocking layer 107 made of p-type AlGaN, a p-type optical guide layer 108 made of p-type GaN, a p-type cladding layer 109 made of p-type AlGaN, and a contact layer 110 made of p-type GaN are sequentially formed from bottom to top on a top surface of an n-type GaN substrate 102. An upper part of the p-type cladding layer 109 and the contact layer 110 form a ridge portion. A p-side electrode 111 is formed on the contact layer 110. An n-side electrode 101 is formed on a back surface (N-terminal face 102A) of the n-type GaN substrate 102.
In order to manufacture the conventional semiconductor device, the n-type GaN layer 103, the n-type cladding layer 104, and the n-type optical guide layer 105 are first sequentially grown on the n-type GaN layer 102 by a metalorganic chemical vapor deposition (MOCVD) method at a growth temperature of 1,050° C. by using a hydrogen carrier gas. The multiple quantum well layer 106 is then grown at a substrate temperature of 700° C. by using a nitrogen carrier gas. The electron blocking layer 107, the p-type optical guide layer 108, the p-type cladding layer 109, and the contact layer 110 are then sequentially grown at a substrate temperature of 1,050° C. by using a hydrogen carrier gas.
The p-type cladding layer 109 and the p-type GaN contact layer 110 are then etched into a stripe shape with a width of 5 μm by using a dry etching device, whereby a ridge-type optical waveguide is formed. The p-side electrode 111 is then formed on the contact layer 110, and the n-side electrode 101 is formed on the N-terminal face 102A of the n-type GaN substrate 102. Finally, cleavage is performed so that the ridge-type optical waveguide has a length of 1 mm, and a dielectric material or the like is deposited on the cleaved surface to form a mirror end face. A semiconductor laser is thus completed.
A p-type nitride semiconductor grown by a MOCVD method usually contains hydrogen atoms having approximately the same concentration as an acceptor concentration. Since the hydrogen atoms hinder generation of holes, the p-type layer has a high resistance right after the growth. Therefore, after the p-type layer is grown by the MOCVD method, a process for reducing the resistance of the p-type layer is performed. This process is called an activation process.
Performing an annealing process at 400° C. or higher as described in Japanese Laid-Open Patent Publication No. H05-183189 is known as a p-type layer activation process. Japanese Laid-Open Patent Publication No. H11-126758 also describes a technology of improving a contact resistance between p-type GaN and a metal by facilitating activation of a p-type layer by irradiation of light with energy exceeding a semiconductor bandgap.